When an ice class ship cruises in group of floating ice pieces or a structure is installed in such field, estimating force acting on the ship or structure by the floating ice pieces is significant for designing them. Several experiments and numerical simulations were conducted to understand the interaction between ship or structure and ice pieces. Among them, Konno et al. (2013) carried out the numerical simulation for calculating resistance on a ship induced by brash ice, and compared it with another experimental result by Wang et al. (2009). The comparison indicated that the numerically simulated resistance differed from the experiment. More specifically, the experimental resistance increased linearly against the carriage velocity, meanwhile the simulated resistance showed quadratic increase. One of the possible mechanisms is treatment of fluid force acting on an ice piece in the numerical simulation. It was based on drag force, which can be applied for high Reynolds number, i.e. the force increases quadratically with the carriage velocity. However, according to the experimental result, fluid force acting on an ice piece in group of multiple ice pieces is not considered to be employed this trend. Therefore, we measured the fluid force acting on a synthetic ice piece surrounded by multiple synthetic ice pieces. From the result, linear relationship was modeled to express the fluid force on an ice piece in group of ice pieces versus carriage velocity.